Introduction

In this chapter, we extend the treatment of rotational excitation, presented in Chapter 4, to encompass vibrational transitions. In practice, the discussion will be limited to linear molecules for which calculations are feasible and quantitative results have been obtained. As compared with rotational transitions, which may be excited at kinetic temperatures T ≈ 10 K, in the case of heavy molecules, or T ≈ 100 K, in the case of light (hydrogen-bearing) molecules, vibrational excitation generally requires T ≈ 1000 K. There are exceptions: the inversion motion (of the nitrogen in the plane of the hydrogen atoms) in NH3 is a form of vibrational motion, and the energy involved is small, of the order of 1 K. Similarly, the torsional motion (of the CH3 relative to the OH group) in CH3OH may be viewed as another form of vibrational motion and involves energies of the order of 100 K. These phenomena have been discussed in Chapter 4. In the present chapter, we shall be concerned with the stretching of chemical bonds within molecules, a process that involves higher energies, on the order of 1000 K.

Vibrational transitions are important in a number of astrophysical contexts, including cool stellar atmospheres, shocked regions of molecular clouds, and planetary nebulae.